A$AP Rocky would get knocked out if he went toe-to-toe with A$AP Relli ... but, he can't touch him 'cause Rocky's a powerful dude -- so, says the prosecutor in the case.

John Lewin -- the deputy district attorney who famously won a conviction against real estate mogul Robert Durst -- spoke to the jury during the rebuttal to Rocky's lawyer Joe Tacopina's closing argument.

Lewin started off his rebuttal by saying Rocky was embarrassed that he got caught talking behind Relli's back ... a look Lewin says isn't very manly.

Relli went to the meeting admitting he wasn't totally sure what was going to go down, Lewin explains ... but, he knows enough to know he can kick Rocky's butt up and down the block -- but he's held back because of the power Rocky wields in the music industry.

Lewin uses hand motions to get his point across ... raising his hand high in the air to denote Rocky's station before bringing his hand way down low to show where Relli's at in the pecking order.

It seems like Lewin's been ready for a fight all day ... getting into a shouting match with Tacopina earlier this morning over the prominent defense attorney's allegations of misconduct.

Joe calling out John's ethics clearly set him off ... and, while it didn't come to blows, it just seems like Lewin's got fisticuffs on the brain.

Testimony ended Wednesday when A$AP Rocky decided not to take the stand in his own defense. Rihanna has been by his side throughout much of the trial ... including bringing the kids to closing arguments on Thursday.